Description of problem:
The 3.2.19 we have as an errata candidate for RHEL-5.3 has the following edge case bug we've just fixed upstream:
https://bugzilla.redhat.com/show_bug.cgi?id=461995
...the problem is that yum will always do some queries via. SQL now, however in some cases SQLite has performance problems. The bug above is for OLPC which is creating exclude lists with 2,200 packages in them, however:
time yum list $(perl -e 'print "python-urlgrabber " x 2200')
...will also hit the same problem.
The speed difference is between ~2.5 seconds for a full repo. load, and upto 5 minutes for the above 2,200 package names. The fix isn't that complicated, so is viable.
It's also worth noting that the speed drop is basically linear with the package list size, so if people are normally only doing 100-200 packages it'll only be a couple of seconds slower you.

An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on therefore solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.
http://rhn.redhat.com/errata/RHBA-2009-0176.html